While much has been learned about neuroprotection—the mechanisms and strategies by which the brain responds to neuronal injury or the early stages of a disease—still more about these abilities remains unknown. Researchers are now pursuing several lines of investigation to understand the nature of neurons’ ability to protect themselves and to recruit this capacity in disease prevention and therapy. The protective influences they are studying range from common molecules in the brain to lifestyle choices such as diet and exercise.

Ever since Italian neuroscientist Rita Levi-Montalcini’s 1952 discovery of nerve growth factor, a molecule that promotes the survival and differentiation of neurons, scientists and drug companies have sought to find and use molecules and chemicals that work similarly to protect the central nervous system. Twenty years ago, investigations on neuronal damage in stroke and spinal cord injury led to studies of growth factors and other ways to protect and repair nerve cells from both kinds of injuries. To date, however, the FDA has approved only two neuroprotective agents: riluzole for amyotrophic lateral sclerosis (ALS) and memantine for moderate to severe Alzheimer’s disease.

Research in 2009 helped to explain some of the mechanisms of neuroprotection, as well as ways to foster greater protection of brain cells and thereby help control the aging process. In particular, scientists this year studied the protective qualities of a family of enzymes called sirtuins, sex hormones such as estrogen, and vitamin D.

Proteins Constantly Protect the Brain

Researchers have found that the brain produces certain chemicals to help protect neurons, including several that encourage neuronal growth and allow some parts of the brain to take over the function, to some extent, of areas damaged by illness or injury. Scientists first identified heat shock proteins, a class of proteins that become especially active when cells are exposed to elevated temperatures and stress, in the 1970s. They have been reported to increase cell survival in response to a wide range of cellular stressors. One of these proteins, heat shock protein-70 (HSP-70), is reported to protect against oxidative stress, an oxygen imbalance that leads to the formation of highly reactive molecules called free radicals.1 Oxidative damage from free radicals has been implicated in a number of diseases, including Alzheimer’s.

Scientists discovered brain-derived neurotrophic factor, or BDNF, in the 1980s. BDNF is a member of the neutrophin family of proteins and acts on certain neurons in the brain and spinal cord, helping to maintain the health of existing neurons and encouraging the growth and differentiation of new neurons and synapses after injury. BDNF is particularly active in the hippocampus, cortex and basal forebrain, areas vital to learning and memory, where neurons regularly make new connections, and, in the case of the hippocampus, new neurons are born throughout life. Studies have linked reduced levels of BDNF to neurodegenerative diseases such as Alzheimer’s and Parkinson’s, suggesting that, in adequate amounts, this natural protein may prevent the cell death caused by those underlying diseases.2

Now scientists are adding more chemicals to their neuroprotection list, including a family of enzymes called sirtuins, which are considered universal regulators of aging in virtually all living organisms, from fruit flies and worms to humans.

Sirtuins have numerous health benefits, including providing protection against neurodegenerative diseases such as Alzheimer’s and ALS, and increasing the number and function of mitochondria (a cell’s energy factory), the dysfunction of which is associated with progressive neuronal degeneration. Caloric restriction activates sirtuin proteins. Resveratrol, an antibiotic produced naturally by some plants when under attack by pathogens such as bacteria or fungi, is a potent activator of SIRT1, one of the seven sirtuin proteins, which also aids in cell survival and neuroprotection.

Christoph Westphal, senior vice president of Sirtris Pharmaceuticals, Inc., a company focused on the research and development of small molecule drugs that target enzymes called sirtuins, and David Sinclair of Harvard Medical School, are researching the health benefits of sirtuins, which affect the genes involved in aging. (Copyright Sirtris, a GSK company, and David Shopper Photography)

In 2006 David Sinclair of Harvard Medical School published two papers showing that resveratrol, which is found in the skin of red grapes and is a major constituent of red wine, could reduce the impact of a high-fat diet, increase stamina and extend the life span of mice. A follow-up study published in Nature by Sinclair’s colleagues at Sirtris Pharmaceuticals demonstrated that novel drug compounds, based on SIRT1, offer a promising new approach to treating agerelated diseases.3

“The new drug candidates represent a significant milestone because they are the first molecules that have been designed to act on genes that control the aging process. For this reason, we feel they have considerable potential to treat diseases of aging,” said Christoph Westphal, who led the research team.

Drinking red wine is considered unlikely to provide the desired neuroprotective effects. In a 2009 paper in Expert Opinion inTherapeutic Patents, however, Francisco Alcain and José Villalba of the University of Cordoba, Spain, pointed out that new SIRT1 activators are up to one thousand times more effective than the resveratrol found in grapes and wine.4 High doses of natural resveratrol might not be sufficient to produce a neuroprotective effect, they said, arguing that scientists need to develop new synthetic sirtuin activators.

Researchers at the MassGeneral Institute for Neurodegenerative Diseases (MIND) reported in the July 2007 issue of Science that blocking the activity of another sirtuin protein, SIRT2, may be an effective therapy in Parkinson’s disease.5 The scientists, led by Aleksey Kazantsev, director of MIND’s drug discovery laboratory, found that blocking SIRT2 activity could protect neurons damaged by the toxic effects of alpha-synuclein, a protein that accumulates in the brains of Parkinson’s patients (see chapter 3, “Parkinson’s Disease”).

Some scientists believe that alpha-synuclein folds abnormally in dopamine-producing neurons in Parkinson’s patients, forming toxicinclusion bodies, clumps of the protein that lodge inside and kill or impair these cells. In 2006, the MIND investigators studied a laboratory chemical called B2 that reduces toxicity in cellular disease models, and found that inhibition of SIRT2 reduced the toxicity of alpha-synuclein. On the basis of the B2 structure, Kazantsev and his team developed a SIRT2 inhibitor, called AGK2, that is ten times as potent as B2. The findings, said Kazantsev, will allow scientists to pursue innovative new drugs to treat and perhaps even cure Parkinson’s and other neurodegenerative diseases.

Vitamin D Protects Cognition

A particularly lively area of neuroprotection research in recent years has been the exploration of evidence that vitamin D (25-hydroxyvitamin D) may have significant neuroprotective features. Vitamin D is found in very few foods. Most people get their intake through sunlight or food supplements, and thus many of the studies have focused on the consequences, particularly in aging, for people with too little of the “sunshine vitamin.”

Low levels of vitamin D are linked to a range of health issues, including weak bones and muscles, certain cancers, high blood pressure and congestive heart failure. Less well known are findings relating insufficient vitamin D to mental illnesses, such as depression and seasonal affective disorder. Studies have also discovered that vitamin D plays a role in neurodegenerative diseases such as Alzheimer’s and Parkinson’s by promoting the production of neurotrophic factors, as well as in cognitive impairment and memory loss.

In 2001 scientists from Taiwan and the National Institute on Drug Abuse reported that vitamin D3, one of the two major active forms of vitamin D, can restore muscle activity in rats induced with Parkinson’s disease. Their study, published in Brain Research, showed that treating the rats with D3 improved muscle movement and reduced the dopamine neuronal toxicity caused by the compound given to induce Parkinson’s.6 The researchers think that the reversal of a toxic mechanism that injures cells via free radicals and reactive oxygen may take place when the D3 is administered.

Studies in 2009 sought to clarify such indications of vitamin D’s neuroprotective role. In a study published in the May issue of the Journal of Alzheimer’s Disease, William B. Grant of the Sunlight, Nutrition and Health Research Center in San Francisco presented an analysis of several large studies showing that vitamin D can help reduce dementia.7

The development of dementia, a long-term decline in cognitive function progressively worsening beyond what is normal in aging, involves several mechanisms, including oxidative stress, inflammation and reduced neurogenesis in the adult brain. Epidemiological evidence suggests that vitamin D reduces the risk of several diseases that are risks for or can precede dementia. “It appears that vitamin D metabolites, especially the active form of vitamin D3, can counter many of the mechanisms linked to risk of dementia,” said Grant.

Another study found that vitamin D may offer some protection against the neurodegeneration seen in Alzheimer’s disease, the most common form of dementia. The researchers reported in the Journal of Geriatric Psychology and Neurology that as vitamin D levels decreased in study participants, all over age sixty-five, levels of cognitive impairment rose. Individuals with the lowest levels of vitamin D were more than twice as likely to be cognitively impaired.8 The association between vitamin D and cognitive impairment was stronger in men. “The cause of dementia is not [simply the lack of] vitamin D,” said David Llewellyn of Cambridge University, the lead author of the study. “It’s a very complicated disease. But while further research is needed, vitamin D supplementation is cheap, safe, and convenient and may therefore play an important role in prevention.”

Adding to this research are findings correlating vitamin D and cognition in middle-aged and older men.9 Scientists at the University of Manchester compared the cognitive performance of more than 3,100 men, ages forty to seventy-nine, at eight European Male Aging Study test centers in Europe. The men with higher levels of vitamin D consistently performed better on simple tests that measure attention and speed of information processing.

In older persons, cognitive impairment decreases as the level of vitamin D in the blood increases. (Courtesy of David Llewellyn)

“The positive effects vitamin D appears to have on the brain need to be explored further, but certainly raise questions about its potential benefits for minimizing age-related declines in cognitive performance,” said lead author David Lee.

Sex Hormones and Neuroprotection

Sex hormones—androgens (male) and estrogens (female)—affect the growth and function of the reproductive organs, development of sex characteristics, and behavioral patterns. Studies of estrogen during the past twenty years have brought to light a remarkable number of ways in which these hormones do far more than serve reproduction. Scientists have come to recognize that estrogen is an important neurotrophic and neuroprotective factor and have implicated it incognition, synaptic plasticity, memory and neurogenesis.

As we age, levels of sex hormones in the blood and brain begin to decline. Many of estrogen’s neuroprotective actions are relevant to Alzheimer’s disease prevention, including reducing beta-amyloid accumulation, a critical factor in Alzheimer’s progression, and reduced plasticity in neuronal dendritic spines, which serve as storage sites for synaptic strength. Studies suggest that reduced estrogen levels in women and age-related testosterone loss in men can contribute to the development of Alzheimer’s disease.

In the late 1990s German scientist Christian Behl discovered that estrogen can also act as an antioxidant. His research demonstrated that in high levels estrogen reduces the neuron-killing effects of free radicals by making free radicals less toxic to neurons.10 Later, Bruce McEwen of Rockefeller University showed that estrogen has the potential to improve mental function by enhancing neuronal survival in the hippocampus, where, his team discovered in rats, estrogen helps to build and maintain new synapses.11

In 2008 Colin Saldanha of Lehigh University discovered that in the brains of birds and mammals that have sustained brain injuries, testosterone is converted into estrogen, a process that can decrease neuronal degeneration and may enhance the recovery of neurons. In the July 2009 issue of Frontiers in Neuroendocrinology, Saldanha and his colleagues reported that the expression of aromatase, an enzyme responsible for a key step in the synthesis of estrogen from testosterone, can raise estrogen levels enough to interfere with apoptosis, or programmed cell death, and ultimately lessen the extent of damage from brain injuries.12 This activity appears to protect neurons by slowing down the degeneration of the damaged cells and increasing the speed with which they are repaired.

Recent data suggest that the elevation of estrogen levels by aromatase selectively activates certain signaling pathways such as the blocker of programmed cell death, Akt. “While a direct link between aromatase and Akt signaling remains to be established, this interaction presents a promising explanation of the neuroprotective effects of brain aromatase,” Saldahna and his colleagues wrote.

One of the most common neurodegenerative diseases, Parkinson’s occurs in greater numbers and may progress more quickly in men than women, suggesting that estrogens may confer some resistance to the development and progression of the disease, according to researchers at Laval University Medical Center in Quebec.13 In the lab, the scientists studied the use of various sex hormones, including estradiol, progesterone and androgens, in a mouse model of Parkinson’s. They found that only estradiol had a neuroprotective effect on the animals’ dopamine.

Christian Pike’s research indicated that estrogens and androgens may play a role in preventing Alzheimer’s disease. (Courtesy of Christian Pike / University of Southern California)

In a review of the research, published in Frontiers inNeuroendocrinology in July 2009, a team of scientists led by Christian Pike of the University of Southern California evaluated evidence that estrogens and androgens may help in the prevention of Alzheimer’s disease.14 They found that experimental studies indicate that estrogens reduce neuron loss in Alzheimer’s and also reduce levels of beta-amyloid, while androgens promote “survival in neurons challenged with AD-related insults and reduction of beta-amyloid levels.” In addition, they found that estrogen prevents beta-amyloid accumulation by regulating programmed cell death.

Pike’s team concluded that androgens protect the brain against Alzheimer’s by promoting the survival of neurons in the hippocampus and cortical regions. They also found that androgens limit the accumulation of beta-amyloid and protect the brain against the formation of tau protein, abnormal amounts of which are found in the brains of Alzheimer’s patients.

The authors noted that current treatment research focuses on the development of compounds called SERMS (selective estrogen receptor modulators) and SARMS (selective androgen receptor modulators) to treat Alzheimer’s. In low concentrations, SERMS have been shown to protect cultured neurons from beta-amyloid toxicity. Androgen-based therapy with SARMS is also drawing considerable interest in the scientific community.

Natural Neuroprotection

In addition to these mechanisms of neuroprotection, simple lifestyle choices—such as eating healthy foods, maintaining a healthy weight and engaging in physical and mental exercise—have also been shown to reduce susceptibility to some brain diseases.

The risk for Parkinson’s disease is moderately lower in people who are physically active, according to a Harvard University study from January 2008, which followed more than 143,000 participants for ten years.15 Similarly, researchers from the Group Health Cooperative in Seattle reported a 40 percent lower incidence of Alzheimer’s among study participants who performed light or moderate exercises, such as walking, dancing, jogging, or swimming, more than three times per week compared with those who did these exercises less frequently.16 The researchers, who followed 1,740 participants and measured their exercise frequency, cognitive function and potential risk factors for dementia, said exercise helps foster healthier blood vessels and better blood flow, which may ward off disease.

In addition, moderation in drinking, abstaining from smoking and maintaining a healthy weight may all help protect neurons as well, albeit through mechanisms that are not yet clear to researchers. At a presentation at the 2008 conference of the American Academy of Neurology, scientists from Mount Sinai Medical Center in Miami reported that Alzheimer’s patients who drank more than two beers per day when young went on to develop the disease nearly five years earlier than those with lighter drinking habits. They also found that patients who smoked more than a pack of cigarettes a day developed Alzheimer’s almost two years earlier than those who smoked less or not all. Another study determined that people with high cholesterol, obesity, or beer bellies in their forties were more likely to develop Alzheimer’s later in life.

Scientists also know that an intellectually and socially active brain is more resistant to disease. In an animal study, University of Chicago researchers discovered that when neurons are active and firing, nearby microglial cells release low levels of tumor necrosis factoralpha (TNF-alpha).17 When released in large amounts, TNF-alpha can kill neurons. The Chicago scientists found, however, that at concentrations well below these lethal levels, TNF-alpha can actually protect neurons from toxic or injury-related damage. TNF-alpha, they said, causes neurons to increase their production of as yet unidentified growth factors that repair DNA and defend against injury.

Conclusion

Although hundreds of molecules—from free radical scavengers and apoptosis inhibitors to neurotrophic factors—have been investigated for their neuroprotective properties, there has been little success in moving these potential treatments from the laboratory to human trials, for a number of reasons. Animal models do not often accurately simulate diseases in humans. The pathophysiology of diseases in humans differs from that in animals. And most laboratory animals have smaller brains than humans.

Despite these challenges, scientists continue to focus their work on advancing neuroprotection as a desired therapy for neurodegenerative diseases and brain injuries. Building on studies from the past two decades, findings from the 2009 research on sirtuins, sex hormones and vitamin D have taken them a step closer to that reality.